1. Field of the Invention
This invention relates to a vinyl chloride resin composition having notably improved compression set and heat deformation resistance and also having a superior tensile strength, wear resistance and processability.
2. Description of the Related Art
Soft vinyl chloride resin is a material affording, when molded, a product having a touch similar to that of rubber, and also having superior processability, physical properties and well-balanced cost. Thus, the resin has been used as a material for automobiles, machines, electrical products, medical products, architecture, etc., in various and broad fields. However, as compared with vulcanized rubber, the material has drawbacks of larger compression set, inferior heat deformation properties, etc.; thus its use has been limited.
As techniques for overcoming such drawbacks, a technique of using a vinyl chloride resin having a higher polymerization degree and a technique of using a partially cross-linked vinyl chloride resin have so far been known. However, according to these techniques, the improvement has been limited, and the compression set is still high and the heat deformation properties are also inferior so that the improvement cannot be regarded as sufficient.
Further, a technique of mixing a cross-linked rubber component with a partially cross-linked vinyl chloride resin has also been known. However, according to this technique, when the rubber component represents a larger proportion, a compression set similar to that of vulcanized rubber is afforded, but with an increase in the content of the vinyl chloride component, the compression set and heat deformation properties are both lower. Thus, in order to afford a compression set matching that of vulcanized rubber, a large quantity of the rubber component is required so that cost increase is inevitable and also the superior properties of the vinyl chloride resin are damaged.
On the other hand, a technique of blending a polyisocyanate compound with a vinyl chloride resin has been known. Japanese patent application laid-open No. Sho 51-31750 (Japanese patent publication No. Sho. 53-42057) discloses a process for improving the compression set and heat deformation properties of a vinyl chloride resin, by blending a polyisocyanate as a crosslinking agent with a vinyl chloride resin obtained according to an emulsion polymerization process and containing a hydroxyl group or a carboxyl group in its molecule. However, according to this process, since the vinyl chloride resin is crosslinked with a polyisocyanate, there are drawbacks that the hardness of the resulting polymer increases, its flexibility decreases, etc. Further, according to the process, while the polymer obtained according to the emulsion process can be processed according to a coater process, the whole of the resulting composition is crosslinked in the case of a more general processing process such as extrusion molding, injection molding, etc., accompanied with a higher temperature kneading; thus only a product having inferior surface properties is obtained.
Japanese patent application laid-open Nos. Hei 1-185313 and Hei 1-254725 disclose a polyvinyl chloride-polyurethane composite material obtained by subjecting a polyol-containing vinyl chloride polymer and a polyisocyanate compound to a urethanization reaction. However, according to this process, the polyvinyl chloride and the polyurethane are only homogeneously dispersed or partly bound to the resulting chain, but the polyvinyl chloride and the polyurethane do not form any co-crosslinked structure; thus no improvement in the compression set and heat deformation properties is exhibited so that such a process cannot be regarded as a suitable process.
Further, Japanese patent application laid-open No. Sho 5-83039 discloses a process of impregnating a polyvinyl chloride resin with a polyol compound and a polyisocyanate compound and forming a polyurethane in the polyvinyl chloride resin. However, according to this process, too, as in the above process, the polyvinyl chloride and the polyurethane do not form any co-crosslinked structure; hence no improvement in the compession set and heat deformation resistance is exhibited so that such a process cannot be rendered as a suitable process.